Optics and Telescopes

Optics and Telescopes: Guiding Questions

1. How do reflecting and refracting telescopes work?2. Why is it important that professional telescopes be

large?3. Why do most modern telescopes use mirrors rather

than lenses? 4. Why are observatories in such remote locations?5. Do astronomers use ordinary photographic film to take

pictures of the sky? Do they actually look through large telescopes?

6. How do astronomers use telescopes to measure the spectra of distant objects?

7. Why do astronomers need telescopes that detect radio waves and other non-visible forms of light?

8. Why is it useful to put telescopes in orbit?

Two Basic Telescope Designs

• Refractors– Use lenses to concentrate incoming

light at a focus.

• Reflectors– Use mirrors to concentrate

incoming light at a focus.

The goal is always the same – gather as much light as possible

and concentrate it at a focus.

Refractor

Reflector

Refraction: Bending of light when propagating into material with higher refractive index (e.g. glass)

A refracting telescope uses a lens to concentrate incoming light at a focus.

Image Formation

Image Formation

Functions of a telescope

• Magnify

A refracting telescope actually uses two a lenses: an objective and an eyepiece.

The two lenses are separated by the sum of their focal lengths.

Magnification of a 2-lens refracting telescope

Eyepiece Objective lens

Example:

FO = 1 m (1000 mm)

FE = 25 mm

Magnification = FO/FE = 1000 mm/25 mm = 40x

Fo

Fe

Functions of a telescope

• Magnifymagnification = (objective lens focal length / eyepiece lens focal length)

• Brighten

Light gathering power: Comparing two telescopes

• Effective collecting area ~ area (d = telescope diameter

• Compare two telescopes: Hubble (d = 2.4m) and Keck (d = 10m)

2

4A d

22

2

10 104 17.42.42.4

4

Keck

HST

Ax

A

Refracting telescopes have drawbacks

Chromatic aberration:

The index of refraction varies with the wavelength of light, so different colors are focused to different places

Chromatic aberration

Special achromatic compound lenses and lens coatings can often fix this aberration.

Lens is achromatic if it bends light at same angle independent of wavelength.Expensive! Very difficult to make large achromatic lenses.

The largest research telescopes

in the world are ALL reflectors.

The Keck I telescope on Mauna Kea on the Big Island of Hawaii uses 36 hexagonal mirrors to make a total diameter of 10 m.

(Note the astronomers standing on either side of the platform.)

A reflecting telescope

uses a mirror to

concentrate incoming light at a

focus.

The secondary mirror in the tube does not cause a hole in the image.

It does however make it slightly dimmer because it reduces the total amount of light reaching the primary mirror.

Drawback of Using Spherical Mirrors in Reflecting Telescope

• Spherical Aberration

(can be corrected with a correcting lens)

A Charge-Coupled Device (CCD)

An electronic device is commonly used to record the image at a telescope’s focus

Ordinary Photographs vs. CCDs

Functions of a telescope

• Magnifymagnification = (objective lens focal length / eyepiece lens focal length)

• Brightencalled light gathering powerProportional to the diameter of the objective lens.

• Resolve fine detailcalled angular resolutionProportional to the size of the telescope (array).

Poor and Good Angular Resolution

Telescope images are degraded by the blurring effects of the atmosphere and the telescope resolution.

Angular resolution

How close can two stars be before they blur into one?

Measured in angular unit, radians or arcseconds.

Diffraction

Waves are bent when they pass through a narrow opening.

This places a limit on the angular resolution of any telescope.

Computing the diffraction limit or angular resolution of a telescope (ignores blurring of atmosphere)

d

where λ, d are in same units (e.g. meters) and θ is in radians

1 radian = 57.296 = 206,265 arcsec (“)

Example: HST, yellow light (λ ≈ 500 nm)

97550 10

2.3 10 0.052.4

mradian

d m

University of Iowa’s Rigel Telescope in Arizona

Spectrographs record the spectra of astronomical objects.

Spectrographs record the spectra of astronomical objects.

Spectrum of Vega (Rigel Telescope)

Electromagnetic spectrum

The “spectrum” of a particular star is how much light it produces at each wavelength.

How can we observe nonvisible light?

• A standard satellite dish is essentially a telescope for observing radio waves

Radio Telescopes

• A radio telescope is like a giant mirror that reflects radio waves to a focus

An example of an interferometer

Very Long Baseline Array (VLBA)

North Liberty Iowa VLBA

Radio Telescope

• 82 ft (25m) diameter

• Built 1991-1992

• Total cost $3M

• Part of VLBA (10 identical telescopes spanning US)

• Operated from VLBA control center (Socorro NM)

• Operates 24/7

• Science research includes stars, black holes, pulsars, cosmology,

Angular resolution of an interferometer

29

6

1 101.2 10 0.0002

8000 8 10

cm mradian

D km m

D

where D is the largest separation between telescopes.

For example the VLBA has telescopes in Hawaii and Virgin Islands (8000 km). At a typical radio wavelngth ≈1 cm the angulat resolution is:

This is the size of a newspaper at the distance to the Moon

(but can’t read newspaper!)

Observations at other wavelengths are revealing previously invisible sights.

UV

Ordinary visible

infrared

Map of Orion region

Telescopes at high altitude or in orbit around the Earth detect radiation that does

not penetrate the atmosphere.

IR & UV Telescopes

• Infrared and ultraviolet-light telescopes operate like visible-light telescopes but need to be above atmosphere to see all IR and UV wavelengths

SOFIA Spitzer

X-Ray Telescopes

• X-ray telescopes also need to be above the atmosphere

Chandra

X-Ray Telescopes

• Focusing of X-rays requires special mirrors• Mirrors are arranged to focus X-ray photons through

grazing bounces off the surface

Gamma Ray Telescopes• Gamma ray

telescopes also need to be in space

• Focusing gamma rays is extremely difficult

Compton Observatory

Supernova Remnant Cas-A at Three Wavelengths

Visible Light ImageRadio ImageX-ray Image

The Entire Sky at the Visible Wavelengths

The Entire Sky at the 21-cm Wavelengths

The Entire Sky at the Infrared Wavelengths

The Entire Sky at the X-ray Wavelengths

The Entire Sky at the Gamma Ray Wavelengths